1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with improved method and apparatus for mounting a top nozzle on the fuel assembly which increase holddown spring reliability and allow the top nozzle to be removed more readily from the fuel assembly.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array of grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends of the assembly are secured to the guide thimbles so as to form the fuel assembly as an integral unit. The respective top and bottom nozzles extend slightly above and below the ends of the fuel rods, capturing the rods therebetween. An example of a nuclear reactor having fuel assemblies of this conventional design is the one described in U.S. Pat. No. 3,235,463 to Sankovich.
During operation of such fuel assemblies in a nuclear reactor such as the one described in the aforesaid patent, the fuel rods may occasionally develop cracks along their lengths resulting primarily from internal stresses. Since the fuel rods are part of the integral assembly of guide thimbles welded to the top and bottom nozzles, it is difficult to detect and remove the failed rods. To gain access to these rods, it is necessary to remove the affected assembly from the nuclear reactor core and then break the welds which secure the nozzles to the guide thimbles. In so doing, the destructive action often renders the fuel assembly unfit for further use in the reactor because of the damage done to both the guide thimbles and the nozzle which prohibits rewelding. In a nonwelded attachment arrangement, such as illustrated and described in British Pat. No. 1,228,610, disassembly of the fuel assembly would prove to be equally difficult. Here, the upper ends of the guide thimbles are fixed in holes formed in the top nozzle by being expanded after insertion to conform to the shape of the holes and annular grooves therein.
In view of the high costs associated with replacing fuel assemblies, considerable interest has arisen in reconstitutable fuel assemblies in order to minimize operating and maintenance expenses. The general approach to making a fuel assemably reconstitutable is to provide it with a removable top nozzle. One commercial reconstitutable fuel assembly construction, similar to that illustrated and described is U.S. Pat. No. 3,828,868 to Jabsen, employs a threaded arrangement for attaching the the top nozzle to the control rod guide thimbles so that the top nozzle can be removed to gain access to the fuel rods contained in the fuel assembly. The top nozzle removal method consists of, first, unscrewing a multitude of threaded connections, for example sixteen, to release the top nozzle from the guide thimbles, and then, pulling the top nozzle over the external threads during removal. The sequence is reversed during top nozzle replacement.
Removal and replacement of the top nozzle from and to the threaded portions of the guide thimbles as well as retightining the threaded connections increases the risk of damaging the threads. Any thread seizure or damage incurred during removal and replacement can ruin the fuel assembly skeleton and require that a new one be substituted for it. Consequently, a need exists for improvement in the method of removing the top nozzle from the guide thimbles and in the apparatus allowing for removability of the top nozzle.